compile(mode='eval') uninformative error message

[ikamensh]

BPO	38131
Nosy	@benjaminp, @ikamensh, @pablogsal, @isidentical
PRs	bpo-38131: Improve messages when generating AST nodes from objects wi… #17715

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-09-12.10:21:59.042>
labels = ['interpreter-core', '3.9']
title = "compile(mode='eval') uninformative error message"
updated_at = <Date 2020-05-07.15:16:43.409>
user = 'https://github.com/ikamensh'

bugs.python.org fields:

activity = <Date 2020-05-07.15:16:43.409>
actor = 'BTaskaya'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2019-09-12.10:21:59.042>
creator = 'Ilya Kamenshchikov'
dependencies = []
files = []
hgrepos = []
issue_num = 38131
keywords = ['patch']
message_count = 3.0
messages = ['352090', '358905', '368351']
nosy_count = 4.0
nosy_names = ['benjamin.peterson', 'Ilya Kamenshchikov', 'pablogsal', 'BTaskaya']
pr_nums = ['17715']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue38131'
versions = ['Python 3.9']

[ikamensh]

While trying to construct a valid ast node programmatically, I have tried following:

import ast

tree = ast.BinOp(left=ast.Num(n=2), right=ast.Num(n=2), op=ast.Add())
expr = ast.Expression(body=[tree])
ast.fix_missing_locations(expr)
exe = compile(expr, filename="", mode="eval")
print(eval(exe))

Unfortunately this gives unhelpful error message:

>>>    exe = compile(expr, filename="", mode="eval")
TypeError: required field "lineno" missing from expr

Turns out I need to make body of ast.Expression not a list, but a node:
expr = ast.Expression(body=tree) # works

Confusion also comes from naming the field "body", which takes value of a list for ast.Module and some others.

[isidentical]

with PR 17715

>>> import ast
>>> expr_without_lineno_but_ok = ast.Expression(body=ast.BinOp(left=ast.Num(n=2), right=ast.Num(n=2), op=ast.Add()))
>>> expr_with_lineno_but_with_wrong_body = ast.Expression(body=[ast.BinOp(left=ast.Num(n=2), right=ast.Num(n=2), op=ast.Add())])
>>> ast.fix_missing_locations(expr_with_lineno_but_with_wrong_body)
>>> compile(expr_without_lineno_but_ok, "<temp>", "eval")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: required field "lineno" missing from expr
>>> compile(expr_with_lineno_but_with_wrong_body, "<temp>", "eval")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: expected some sort of expr, but got [<_ast.BinOp object at 0x7f3f57dc4c80>]

[isidentical]

I've updated PR 17715 in order to point out exact field that error happened. Also it now only shows type

>>> import ast
>>> expr_without_lineno_but_ok = ast.Expression(body=ast.BinOp(left=ast.Num(n=2), right=ast.Num(n=2), op=ast.Add()))
>>> expr_with_lineno_but_with_wrong_body = ast.Expression(body=[ast.BinOp(left=ast.Num(n=2), right=ast.Num(n=2), op=ast.Add())])
>>> ast.fix_missing_locations(expr_with_lineno_but_with_wrong_body)
<ast.Expression object at 0x7f854123fbb0>
>>> compile(expr_without_lineno_but_ok, "<temp>", "eval")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: required field "lineno" missing from expr
>>> compile(expr_with_lineno_but_with_wrong_body, "<temp>", "eval")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: For field 'body'; expected expr type node, got list